In paper and tissue web drying operations, it is often desirable to remove water by evaporation following initial dewatering steps which remove water from the paper fiber by mechanical means. Typical mechanical dewatering is carried out in a Fourdrinier machine or similar device, wherein one or more suction boxes are arranged to pull an air vacuum through a traveling wire or fabric belt while the wet paper fiber slurry is carried on the opposite surface of said fabric belt in relation to the suction boxes. This mechanical dewatering step is typically not capable of removing sufficient water to meet the requirements of the final moisture content of the paper or tissue product. Typically, additional moisture is required to be removed by evaporation in one or more drying steps.
One conventional method for drying a continuous web of uncoated or unsized paper, including tissue, uses cast iron dryer cans or larger structures called “Flying Dutchman” or “Yankee Dryers,” both of which are also cast iron drums. All of these conventional cast iron drums are rotating devices wherein the web to be dried is brought into contact with a heated surface. Heat is thus conducted to the web directly but the solid surface of the drum blocks mass transfer by convection on the side in contact with the drum. Mass transfer occurs only on the side opposite the contacting surface. This effectively limits the drying rate that could otherwise be achieved if one side were not blocked from mass transfer by the drum surface.
It is known to those skilled in the art that a continuous web may be dried simultaneously from both sides by means of hot air impingement nozzles positioned on both sides of a web. Heat and mass transfer may be brought to both sides of the web by a type of impingement dryer which supports the web using flotation nozzles or “air bars” as they are referred to by those skilled in the art.
One conventional arrangement for contactlessly supporting a web during drying includes horizontal upper and lower sets of air bars between which the web travels. Hot air issuing from the air bars both dries and supports the web as it travels through the dryer. The air bar array is typically inside a dryer housing which can be maintained at a slightly sub-atmospheric pressure by an exhaust blower that draws off the moisture or other volatiles emanating from the web as a result of the drying of the water, coating or ink thereon, for example.
It would be desirable to utilize air flotation to convey and dry wet tissue webs at the high speeds associated with tissue manufacture, such as tissue grade paper including bath or facial tissue and towel products. However, air bar arrangements in conventional flotation dryers are designed to float a continuous web under tension without support from a belt. In most cases tissue or light paper is not strong enough to sustain the web tension necessary for conventional flotation, therefore it is desirable to retain the belt for support. However, conventional flotation dryers exhibit insufficient web handling characteristics when a web such as tissue or light paper is carried on a fabric support belt. Experiments carried out in a pilot dryer with belt support showed excessive movement of the tissue on the belt leading to web billowing and lateral bunching, or narrowing (roping) leading to web breaks when conventional air bars were run at air velocities needed to dry at the evaporation rates necessary for tissue or paper production rates to be commercially successful. Air velocities above 4000 feet per minute were problematic to web handling, yet air velocities in excess of 10,000 feet per minute are most desired for sufficiently high heat and mass transfer to support drying rates needed for economical production of tissue and paper.